The present invention relates to a method of injection molding an improved preform for use in blow molding a container such as a beverage bottle having a bottom with improved structural strength and durability.
More specifically, the present invention relates to a method of injection molding a multi-layer preform from a combination of virgin and recycled plastic such as PET so that the resultant blow molded bottle has a reduced tendency for bottom failure in spite of the use of the recycled plastic.
It is desirable to be able to fabricate multi-layer plastic beverage bottles from the combination of virgin and recycled plastics, such as PET. Ideally recycled PET comprises a central layer of a three-layer bottle structure wherein the inner and outer layers both comprise a higher grade of more expensive virgin PET, so that neither the beverage in the bottle nor the consumer comes into direct contact, with the recycled plastic (PET).
Typically the quality and strength of plastic is measured by its intrinsic viscosity (IV). For example, blow molded plastic PET bottles are generally fabricated from virgin PET having an IV from about 0.70 to about 0.86. On the other hand recycled PET generally has an IV less than 0.74 and from about 0.63 to 0.72.
The lower intrinsic viscosity, recycled PET, is less durable than the higher IV, virgin PET. Therefore, it is subject to a higher probability of creep, stress cracking, impact failure, burst failure or cracking from various other causes. This problem is particularly acute in the bottom region of the bottle which is subject to more wear and tear.
In stretch blow molding plastic containers a preform is typically injection molded and thereafter internally pressurized within a blow mold in order to form a bottle. The sidewalls or body of the preform are materially expanded during the blow molding process causing biaxial orientation of the plastic and increased strength. However, the bottom region of the preform is not materially expanded to the same extent and, therefore, does not undergo biaxial orientation to any significant degree that will result in increased strength. Therefore, regardless of the type of material utilized, the bottom region of the bottle is inherently weaker than the sidewall regions forming the body.
In conventional blow molding the preform mold cavity is filled with injection material through the center point of the base portion of the preform through the injection gate. Typically, crystalline sections (sometimes referred to as "slugs") are produced at the gate after injection molding because there is a steep gradient of temperature at this point caused by the hot (melt) to cold (solidified plastic) interface. This gate crystallinity not only is more brittle than is amorphous or non-crystalline PET, but it also creates high stress concentrations in the base of the final bottle since crystalline material does not stretch and orient during the orientation blow molding process and since the differential densities between adjacent crystalline and amorphous sections induces high residual stresses in the molded preform. This problem becomes even more acute when using a low intrinsic viscosity (IV) plastic since it is subject to a higher crystallization rate and more of the cracking failure problems than a plastic of a higher IV.
Accordingly, a need in the art exists for a method of injection molding a preform which substantially eliminates the presence of "slugs" in the bottom region of the preform in order to achieve a resultant blow molded bottle with a more durable bottom structure.